1. Field of the Invention
The present invention relates to a programmable control system including a plurality of programmable controllers (or programmable ladder controllers, hereinunder referred to as "PC's") for automatically controlling various pieces of equipment.
2. Description of the Related Art
PC's are used for controlling various pieces of equipment such as a transfer machine and a PC having more functions and easy to use has recently been demanded for realizing factory automation.
As shown in FIG. 1, a conventional PC 10 includes a memory 12 for storing a sequence program and a control processor 14 for controlling a piece of equipment in accordance with the sequence program. The PC 10 further includes I/O portion (not shown) connected to an external piece of equipment, data memory (not shown), system memory (not shown), etc. FIG. 2 shows what is called a relay ladder for showing the order and the conditions of control with respect to the equipment being controlled. Equipment being controlled includes virtual equipment (dummy equipment) which exists only in a program.
A relay symbol 16 in the relay ladder diagram represents a piece of equipment being controlled. An address 18 shown in the vicinity of the relay symbol 16 is a local I/O address which is only effective in the PC 10. The I/O address is expressed by an octal digit, a hexadecimal digit or the like and individually allotted to each piece of equipment.
The order and the conditions of control shown in the relay ladder diagram are programmed by a programming language such as a logic symbol language and a relay symbol language and stored in the memory 12 as a sequence program. To state this more concretely, in the sequence program, a command and an I/O address to each piece of equipment are entered, as shown in FIG. 1. The sequence program memory 12 therefore includes a region 12a for storing a command and a region 12b for storing an address.
A conventional programmable control system is composed of a plurality of PC's which are connected to each other through a link bus. The PC's control a plurality of pieces of equipment in parallel with each other while transmitting and receiving link data between PC's through the link bus. The link data is information necessary for successively controlling the PC's. It is, for example, interlock information. In a system including a plurality of PC's, interlock information is essential.
Conventionally, the operator judges what kind of link data is necessary between PC's and writes the number of the PC to which the link data is supplied into, for example, a link control table. In the link control table, the numbers of the PC's to which the link data is supplied are stored in correspondence with the respective steps of the sequence program.
It is therefore necessary for the operator to create a link control table for each PC in planning a system. The manual creation of the link control table is very troublesome and is apt to cause an error.
Especially, in the system including a multiplicity of PC's, it takes the operator much time and labor to create a link control table for each PC, which leads to a trouble in planning and correcting a system.